1. Field of the Invention
The present invention relates to an optical recording medium for recording and reproducing information in a near-field optical recording and reproducing system by the use of an objective lens of which numerical aperture is greater than 1 and an optical recording and reproducing method using this optical recording medium.
2. Description of the Related Art
Various kinds of optical recording mediums typically represented by a CD (Compact Disc), an MD (Mini Disc) and a DVD (Digital Versatile Disc) have been so far widely used as storage mediums for use in storing a variety of information such as music information, video information, data and programs. However, as music information, video information, data and programs are becoming higher in sound quality, are becoming longer in recording and playing time and they are becoming large in capacity, a demand for realizing an optical recording medium having a larger storage capacity and a demand for realizing an optical recording and reproducing device for recording and reproducing this optical recording medium are further increasing.
Therefore, in order to satisfy such demands, in various kinds of optical recording and reproducing device, its light source, for example, its semiconductor laser has been reduced in wavelength and a numerical aperture of its objective lens has been increased, whereby a spot of a light beam focused by the objective lens is reduced in diameter.
With respect to semiconductor lasers, for example, a GaN semiconductor laser with its oscillation wavelength reduced to the wavelength region of 400 nm from 635 nm of a red laser has been put into practical use, and hence a beam spot of a laser beam can be reduced in diameter.
So far there has been examined a so-called near-field optical recording and reproducing system that constructs an objective lens with a numerical aperture greater than 1 by using an optical lens with a larger numerical aperture typically represented by an SIL (Solid Immersion Lens), for example. This near-field optical recording and reproducing system is able to reproduce information by decreasing a space between the opposing surface of this objective lens and an optical recording medium up to approximately a wavelength of a light source or approximately one-tenth of the wavelength of the light source.
FIG. 1 of the accompanying drawings is a schematic diagram showing an arrangement of an example of an optical system of a near-field optical recording and reproducing system according to the related art. As shown in FIG. 1, this near-field optical recording and reproducing system is realized by an objective lens 23 composed of a first optical lens 21 made of a high refractive index material, for example, and which is finished like a super-hemispherical shape and the like and a second optical lens 22 made of a suitable lens such as a convex lens, the first optical lens 21 and the second optical lens 22 being arrayed from the side of an optical recording medium 10, in that order (see cited non-patent reference 1, for example).
In order to increase a recording density in this near-field optical recording and reproducing system, similarly to the related-art optical reproducing system, a wavelength of light emitted from its light source should be decreased and a numerical aperture of its objective lens should be increased, thereby reducing a beam spot of its focused laser beam in diameter.
An area of a beam spot of a laser beam is in inverse proportion to the square of a numerical aperture of an objective lens, and hence in order to realize a high recording density in the near-field optical recording and reproducing system, it is effective to increase a numerical aperture of an objective lens.
As in the example that has been described so far with reference to FIG. 1, for example, a numerical aperture NA of a near-field objective lens obtained when the first optical lens 21 is composed of the super-hemisphere optical lens is expressed by the following equation:NA=nL1×nL1×sin θAIR Where nL1, represents the refractive index of the first optical lens 21 and θAIR represents the angle of incidence of light L incident on the first optical lens 21 in the air (see cited non-patent reference 1, for example).
Accordingly, it becomes necessary to increase the refractive index of the first optical lens in order to increase the numerical aperture of the objective lens.
However, even when the refractive index of the first optical lens 21 increases and the numerical aperture NA of the objective lens 23 increases, satisfactory recording and reproducing characteristics cannot be obtained so that it is difficult at present to realize a high recording density and a large storage capacity in the near-field optical recording and reproducing system.
At the same time, in the near-field optical recording and reproducing system in which the optical recording medium and the objective lens are located close to each other by a space up to several 10 s of nanometers, the surface of the optical recording medium should be made clean and should have very flat properties. In particular, when the near-field optical recording is effected, after recorded pits have been formed with illumination of laser beams, the surface of the optical recording medium should be prevented from being deformed in a convex fashion and a substance on the surface of the optical recording medium should be prevented from being evaporated and attached to the surface of the objective lens.
Further, since the diameter of the beam spot of the laser beam focused by the objective lens is extremely small, it is requested that the optical recording medium should be made of a material that is durable to an extreme rise of temperature on the surface of the optical recording medium.
Furthermore, since general consumers becoming to consume and waste a large amount of recording-type mediums such as an MD and a DVD as personal computers are being widespread in recent years, in consideration of loads to the natural environment, next-generation optical recording mediums should be of optical recording mediums made of materials that are not dangerous and harmful for circumstances and which exert small loads upon natural environment, that is, materials that can be recycled.
[Cited Non-Patent Reference 1]
T. D. Milster, “Chromatic Correction of High-Performance Solid Immersion Lens Systems”, Jpn. J. Appl. Phys., March 1999, Vol. 38, Part 1, No. 3B, pp. 1777 to 1779.